The etiologic agent of Chagas' disease is an obligate intracellular protozoan parasite, Trypanosoma cruzi. In mammalian hosts T. cruzi cycles between a trypomastigote stage which circulates in the blood and the amastigote stage which replicates in the cytoplasm of infected host cells (primarily muscle). Chagas' disease is prevalent in almost all Latin American countries including Mexico and Central America, where approximately 18 million people are infected with T. cruzi and roughly 50,000 children and adults die of chronic Chagas' disease every year due to lack of effective treatments. More than 90 million are at risk of infection in endemic areas. Additionally, 2-5% of fetus carried by infected mothers in endemic areas are either aborted or born with congenital Chagas' disease. Loss of revenue in terms of productivity lost due to sickness and medical costs have an overwhelming effect on economic growth of these countries. In the U.S., 50-100 thousand serologically positive persons progressing to the chronic phase of Chagas' disease are present, and the number of infected immigrants in developed countries is increasing. Therefore, the risk of transmission of T. cruzi to non-infected individuals through blood transfusion and organ transplants from the infected immigrant donors exists.
Attempts to control the vector have been made in an effort to control or prevent T. cruzi infection. Government funded programs for the reduviid vector control and blood bank screening in the developing South American countries have been effective in reducing the transmission of T. cruzi. However, the operational costs to maintain such control programs, behavioral differences among vector species, the existence of animal reservoirs, and the persistence of parasites in chronically infected patients prevent these control measures alone from completely controlling T. cruzi infection.
Chemotherapeutic treatments, a potential means by which parasite load in the acute or chronic phase of the disease development and thereby the severity of disease can be reduced, have been partially successful in controlling T. cruzi infection and Chagas' disease. However, the high toxicity of drugs and poor efficacy of available therapeutics have combined to limit the utility of chemotherapy for treatment of both acute and chronic patients. Further, drug therapy reduces the severity of disease in chronically infected individuals, but cannot reverse the damage already done by parasites.
Vaccines for prevention or treatment of T. cruzi infection are practically non-existent. Traditional vaccines constituted of heat-inactivated parasites, or subcellular fractions of T. cruzi provide a degree of protection from T. cruzi infection (M. Basombrio, Exp. Parasitol. 71:1-8 (1990); A. Ruiz et al., Mol. Biochem. Parasitol. 39:117-125. (1990)). However, these vaccines failed to elicit the protective level of immunity, probably due to loss of important epitopes during inactivation and/or the failure of the antigens to enter the MHC class I pathway of antigen processing and presentation and elicit cell mediated immune responses (J. Monaco, Immunol. Today 13:173-179 (1992)). Live attenuated vaccines are capable of entering the MHC class I pathway, and might elicit protective immune responses. However, the danger of reversion of attenuated parasites to virulent strains if attenuation is not complete renders these vaccines impractical. A DNA vaccine containing the gene encoding a trans-sialiadase has been shown to provide prophylactic protection against T. cruzi infection in mice (F. Costa et al., Vaccine 16:768-774 (1998)), but has not been shown to prevent or reverse disease or to stimulate a CD8+ T cell response in the animal. In another report, specific cellular and humoral immune response in BALB/c mice immunized with an expression genomic library of T. cruzi was observed (E. Alberti et al., Vaccine 16:608-612 (1998)).
Most vaccine research has centered on attempts to develop prophylactic protein vaccines against T. cruzi infection, and has met with little success. The development of subunit vaccines composed of defined antigens which are capable of inducing strong humoral and type 1 T cell responses and reducing the parasite burden has been hindered by the lack of knowledge of the biology of the three developmental stages of T. cruzi, the lack of sufficient sequence information on genes expressed in the infective and intracellular stages, and the prevailing scientific view that chronic disease is not associated with persistent parasitic infection but is the result of a parasite-induced autoimmune response. The presence of polyclonal activation of B and T cells during the acute phase of infection, the difficulty in demonstrating the existence of T. cruzi in the hearts of hosts with severe cardiac inflammation, and the presence of antigens that are shared or cross-reactive between heart and parasites have been used to promote the idea that anti-heart autoimmune lymphocyte cytotoxicity or humoral immune reactions are responsible for the development of Chagas' disease. A corollary to this view is that vaccination against T. cruzi infection or boosting the immune response of infected individuals will exacerbate the disease. On the other hand, immunohistochemical detection of the T. cruzi antigens or detection of T. cruzi DNA by sensitive in situ PCR or reverse transcriptase (RT)-PCR techniques in chronic chagasic cardiopathy in murine models (Y. Gomes, Appl. Biochem. Biotechnol. 66:107-119 (1997); E. Jones et al., Am. J. Trop. Med. Hyg. 48:348-57 (1993); M. Reis et al., Clin. Immunol. Immunopathol. 83(2):165-172 (1997)) as well as humans (J. Lane et al., Am. J. Trop. Med. Hyg. 56:588-595 (1997)) has been reported. Also, a direct correlation between myocardial inflammatory infiltrates and the presence of parasites and development of chronic heart failure in a murine model using heart transplantation (R. Tarleton et al., Proc. Natl. Acad. Sci. USA. 94:3932-3937 (1997)), and in chagasic patients using endomyocardial biopsies (M. Higuchi et al., Clin. Cardiol. 10:665-670 (1987)) has been demonstrated. See R. Tarleton et al., Parasitology Today 15:94 (1999) for a review.